The AI itself wouldn't need to be much smarter, it just needs to be tweaked a little and managed better. Easy AIs could ignore any sounds short of gunshots and need you to move faster to see you in the dark, while hard AIs would have their senses not as limited. Easy AIs could be given equipment that makes them easier to fight for you (if you favor taking out targets from a distance, it could spawn more guards with shotguns), but Hard AIs could adapt to your tactics themselves (so in that case they would have better ranged weapons, smoke grenades to cover themselves, and stay closer to cover).